1. Field of the Invention
The invention relates to removing water vapor trapped in integrated circuit (IC) packages, and more particularly, to a method and system which removes trapped water vapor in IC packages immediately prior to reflow soldering of the IC onto a circuit board as part of the in-line assembly process.
2. Description of the Related Technology
In the surface mount board manufacturing industry, high speed automated placement machines are used to place surface mount components onto bare printed circuit boards (PCBs) prior to their fixation to the board using methods such as the well known reflow soldering process. To accomplish this placement, parts are first received from a manufacturer in one of several mediums, including 1) loose parts in a tube; 2) parts in a tape and reel; and 3) parts trays. These parts are extracted from their packaging mediums and then loaded into a placement machine which automatically places them onto the PCB. One of the most popular packaging mediums is the parts tape and reel because it provides maximum efficiency in retrieving components from the tape and reel and thereafter placing them onto designated PCBs.
FIG. 1 illustrates a typical parts tape 101 in which IC components may be received prior to their assembly on the PCB. As shown in FIG. 1, the parts tape 101 is of sufficient thickness that it includes cavities 103 therein for storing an integrated circuit (IC) component (not shown). These components are sealed or encapsulated within the cavities 103 by means of a covering film 105, typically made of a plastic or other cellophane-type material. The parts tape 101 is typically wrapped around a reel (not shown).
FIG. 2 shows the parts tape 101 of FIG. 1 being fed through a feeder 107. As the parts tape 101 moves through the feeder 107, a component (not shown) within a cavity 103 of the parts tape 101 reaches a pickup point 109 where the component is extracted from the tape 101 either manually or automatically by a component placement ("pick and place") machine. The automatic placement machine typically includes a processor for processing input coordinates in order to accurately determine the position of components during the retrieval from a specified packaging medium, e.g., the parts tape and reel. These placement machines can also calculate the position in which these components are to be placed on a PCB. The placement machines also typically include a controller for controlling a robotic mechanism which picks up each component and then places the component onto the PCB. The automatic placement of a part onto a PCB may be accurate to within fractions of a millimeter of a specified target location. This accuracy is accomplished by a machine vision system which looks for two fiducial coordinates, which are typically actual pads positioned on the PCB. The placement machine then aligns each part or component to be placed on the PCB in relation to these fiducial coordinates. Such automatic placement machines, and their corresponding vision systems, are well-known in the art.
IC packages, as well as other types of plastic packages, absorb moisture from the atmosphere and trap the moisture within tiny pores in each package. This trapped moisture, or water vapor, remains absorbed in the package until it is heated, where upon the water vapor turns into a gas and evaporates out of the pores of the package. It has been observed that this trapped water vapor in the IC package poses a problem during the mounting of the IC onto the PCB.
Surface mount technology (SMT), otherwise known as reflow soldering, is increasingly being employed as a cost-effective means of mounting IC devices to printed circuit boards. Numerous different techniques for mounting integrated circuit devices to circuit boards, chip carriers, or other components, fall within the general category of SMT. In a typical SMT process, an IC device is placed onto and aligned with a printed circuit board (PCB) as was described above, and thereafter mounted to the PCB using reflow solder processing techniques. Reflow solder processes generally use forced convection heating (air or nitrogen) to melt solder interposed between the surfaces to be joined. The IC device and PCB assembly is then exposed to a temperature profile which results in reflow of the solder. Surface tension created in the resulting solder liquid mass during reflow tends to prevent collapse of the solder, causing the joint to eventually solidify in a barrel or truncated sphere shape that is commonly referred to as a controlled collapse chip connection or "C-4" connection. Numerous variations on this general theme exist, including the use of two or more different solders with various melting points to produce reflow of various portions of the joint during different processes, or to allow rework.
Attachment of an IC device onto a PCB is typically accomplished by using solder paste bricks which are formed by screen printing eutectic solder paste onto an array of solder pads on the PCB. A typical method of screen printing includes the step of placing a solder paste stencil, having apertures that are of the same shape as the desired solder paste bricks, onto a PCB such that the apertures are aligned with the pads of the PCB. Solder paste is then applied to the unmasked areas, and the stencil is subsequently removed. The IC device is then placed onto the PCB, typically by an automated placement machine, such that the leads of the IC device make contact with the solder paste bricks on the etched pads of the PCB. The entire assembly is then passed through a reflow process which applies a predetermined time/temperature profile to the solder paste bricks to liquefy the solder paste bricks, thereby forming solder joints between the IC device and the pads of the PCB. The temperatures required to perform this reflow process are in the range of 180-200.degree. C.
At these higher temperatures, any moisture which is absorbed and trapped in the pores of the IC packaging, almost instantaneously transforms into a gaseous state which rapidly expands out of the pores of the IC packaging. The resultant volatile gases literally explode out of the pores of the IC package, thereby creating the "popcorn effect" which can substantially damage not only the package of the IC but the underlying components as well. This rapidly expanding volatized gas sometimes causes problems in the structural integrity of the IC component, such as delamination, cracking or possibly deformation of the package, and hence the reliability and functionality of the IC component is compromised.
Component manufacturers have attempted to remove water vapor from the IC packages by baking the components at a predetermined temperature for a predetermined length of time, e.g., 160 hours at 40.degree. C., or 24 hours at between 90.degree. C. and 100.degree. C. After this baking period, the components are typically encapsulated in a parts tape and reel, parts tray, vacuum-packed bag or vapor barrier bag accompanied by a desiccant package.
Component manufacturers who follow the above process of eliminating or reducing moisture absorbed into the packaging of their manufactured components have not been completely successful. This is because there is typically a lag time between the baking process and the process of packaging the components into the respective packaging media, e.g., parts tape, parts tray or vacuum/vapor barrier bag. During this intermittent delay period, moisture from the atmosphere can settle onto the component packages and be absorbed into the micropores of the packages. Although the bake process eliminates much of the water molecules trapped in the component packages, depending on the delay between baking and sealing in their respective packaging mediums, a significant amount of moisture may once again permeate into the micropores of the packages. Thus, it has been a common experience among electronics manufacturers who buy components from component manufacturers, that many of the components shipped by the component manufacturers contain trapped moisture in their packages. As explained above, this trapped moisture may damage the IC during reflow soldering of the IC onto a circuit board by producing a "popcorn effect" as the heat involved in reflow soldering converts the water molecules into volatile gases which literally explode out of the micropores of the packaging of the IC.
Electronics manufacturers have attempted to deal with the "popcorn effect" by performing an in-house baking process themselves prior to reflow soldering of the components onto PCBs. However, as explained in further detail below, even this second in-house baking process has been unsuccessful in completely eliminating the "popcorn effect" because the electronics manufacturers typically perform their in-house baking process well in advance of the automated assembly process. This delay between the baking process and the reflow soldering process allows moisture from the atmosphere to once again settle into the pores of the component packages.
Prior parts tapes and reels have not been well suited for baking in thermal ovens for prolonged periods of time because these prior parts tapes and reels were typically made from a plastic material which tended to melt, warp, or otherwise become deformed when exposed to temperatures up to 100.degree. C. for prolonged periods of time. As a result, electronics manufacturers have typically extracted the components from the parts tapes and placed them in a tray or other suitable medium having the required thermal characteristics to withstand baking at temperatures of up to 100.degree. C. After the components are baked in this baking tray for approximately four hours, for example, the components are then placed back into the parts tape where they await to be assembled onto designated PCBs. This process of removing the components from the parts tape to place them onto a bake tray and, thereafter, inserting the components back into the parts tape for subsequent assembly by an automated pick and place machine, is a time-consuming and tedious process which wastes valuable resources of the company. Additionally, the amount of time required to re-insert the components from the bake tray into the parts tape allows moisture from the atmosphere to once again be absorbed into the component packages. Furthermore, any additional delay between the time the components are reinserted into the parts tape and the time when they are retrieved from the parts tape by the automated placement machine for assembly onto PCBs, allows additional moisture from the atmosphere to permeate into the component packaging. Therefore, even those electronics companies which perform a second in-house bake process, as described above, prior to assembly of the components onto circuit boards, may experience a significant amount of damage to their components as a result of the "popcorn effect" during reflow soldering of the ICs onto the PCBs.
In view of the deficiencies of prior methods and systems which attempted to eliminate or reduce the "popcorn effect" exhibited during the reflow soldering process, an improved method and system is needed which eliminates the unnecessary time, energy and human resources required to transfer components from a parts tape and reel to a bake tray and thereafter to transfer the components from the bake tray back into the parts tape for subsequent assembly by an automated pick and place machine. Furthermore, the method and system should eliminate or reduce as much as possible any delay between the time the components are baked and the time the components are reflow soldered onto designated PCBs.